Heat pump fluid heating system

ABSTRACT

A heat pump system for raising the temperature of a fluid includes a compressor for compressing a working fluid; a desuperheater heat exchanger provided with an inlet and outlet for a fluid to be heated and an inlet and outlet for the working fluid, the working fluid inlet being communicated with an outlet from the compressor; a condenser heat exchanger provided with an inlet and outlet for the fluid to be heated and an inlet and outlet for the working fluid, the condenser heat exchanger fluid outlet being communicated directly with the desuperheater heat exchanger fluid inlet, and the condenser heat exchanger working fluid inlet being communicated directly with the desuperheater heat exchanger working fluid outlet; and an evaporator with an inlet communicated with the condenser heat exchanger working fluid outlet, and an outlet communicated with an inlet to the compressor.

TECHNICAL FIELD

This invention relates to a heat pump fluid heating system for producinghot fluid at temperatures at least equal to the condensing temperaturein a heat pump system. In particular, the present invention relates to aheat pump fluid heating system for producing hot water at hightemperatures, suitable for use as a processing heat source such as in amilk pasteurizing system.

BACKGROUND ART

Heat pump fluid heating systems are used for example to heat water forvarious applications such as for domestic hot water, or swimming pools.

These systems generally utilize a heat pump cycle using a compressor, acondenser, and evaporator. In the case of domestic water heating wherehigher temperatures are required, the water may be heated to a hightemperature using the superheat from the superheated working fluidexiting the compressor.

U.S. Pat. No. 5,901,563 to Yarbrough et. al. discloses a heat pump heattransfer system which includes a refrigerant to water heat exchanger,known in the art as a desuperheater, for transferring superheat from thecompressed gas exiting the compressor to a domestic hot water service.This enables higher temperatures to be reached as required for domestichot water systems. However, water is only heated at the desuperheater,and while a high temperature can be obtained, the flow rate is small.

For other applications such as for a processing heat source however,heat pumps have had little application, due to their inability toproduce useful flowrates at the required higher temperatures, stemmingfrom the fact that the flow of fluid to be heated (referred to hereunderas heated fluid) necessary for the working fluid condensation isconsiderably greater than is required to de-superheat the same workingfluid, yet only the latter phase possesses the capacity to raise theheated fluid to higher temperatures. This imbalance results in eitherthe provision of a full heated fluid flow at generally lowertemperatures, or as with Yarbrough, a small flow at a highertemperature. In this case, the lower temperature balance is of little orno value, unless low temperature applications are available.

FIG. 1 shows a conventional heat exchanger configuration for hot gascooling of a heat pump system. With this configuration, a heat exchanger1 is configured with a working fluid inlet 2 and outlet 3, and a coolant(heated fluid) inlet 4 and outlet 5. This configuration provides areasonable output flowrate, but only at medium temperatures, beingunsuited to most requirements for high temperature heated water.

The problem of obtaining higher flow rates for a high temperature systemis somewhat overcome by U.S. Pat. No. 4,474,018 to Teagan whichdiscloses a heat pump system for production of domestic hot water, whichinvolves using a compressor section which provides working fluid in amultiplicity of pressures. With this arrangement, water is heated inseries connected heat exchangers, each provided with condensing coils inseparate loops. Having the condensing coils in separate loops enablesthe plant to be designed for optimum performance, since flow rates andtemperatures can be varied for the separate loops. With this design eachof the heat exchanger/condensor sections combine desuperheating andcondensing, and are in effect the same as shown in FIG. 1. While havingseparate loops enables design for optimum performance, this adds to thecomplexity of the system and hence cost and size.

Furthermore, neither of the above patents disclose the use of aliquid/gas heat exchanger to improve the system economy by transferringheat between the working fluid output from the condenser and the workingfluid input to the compressor. Nor do they disclose the possibility ofalso using the heat pump to concurrently provide chilled water, such asis required for example in a milk pasteurizing plant.

DISCLOSURE OF INVENTION

It is an object of the present invention to address the above problems,and provide a heat pump fluid heating system which enables a compactdesign, and which can achieve sufficient flows of high temperature fluidfor use in processing plants such as for sterilizing, and pasteurizing.

Moreover it is an object to provide a method of determining the requiredheated fluid mass flow rate and heated fluid entering temperature forsuch a heat pump fluid heating system.

According to one aspect of the present invention there is provided aheat pump system for raising the temperature of a heated fluid,comprising;

a compressor for compressing a working fluid,

a desuperheater heat exchanger provided with an inlet and outlet for theheated fluid and an inlet and outlet for the working fluid, the workingfluid inlet being communicated with an outlet from the compressor,

a condenser heat exchanger provided with an inlet and outlet for theheated fluid and an inlet and outlet for the working fluid, thecondenser heat exchanger heated fluid outlet being communicated directlywith the desuperheater heat exchanger heated fluid inlet, and thecondenser heat exchanger working fluid inlet being communicated directlywith the desuperheater heat exchanger working fluid outlet, and

an evaporator with an inlet communicated with the condenser heatexchanger working fluid outlet, and an outlet communicated with an inletto the compressor.

The compressor may be any suitable device such as a rotary compressor, ascrew compressor or a reciprocating compressor, in either single ormultiple stages. Moreover, two or more compressors may be provided asrequired.

The evaporator may be any conventional evaporator used for a heat pumpsystem, such as an air cooled or liquid cooled evaporator. In the casewhere process cooling is also required, the evaporator may be a liquidcooled heat exchanger adapted for connection to a liquid recirculationsystem, for providing cooling.

The desuperheater heat exchanger and the condenser heat exchanger may bearranged in any suitable configuration, provided these are connected inseries. For example the desuperheater heat exchanger may be arrangedabove the condenser heat exchanger so that any condensate from thedesuperheater heat exchanger will flow down into the condenser heatexchanger.

In a preferred embodiment, where economy of space is a prerequisite, thedesuperheater heat exchanger may be arranged so that a working fluidoutlet therefrom is below an inlet to the condenser heat exchanger, andthere is provided a device for carrying any condensate into thecondenser heat exchanger inlet.

With this arrangement, the desuperheater heat exchanger and thecondenser heat exchanger may be arranged side by side, thus providing acompact arrangement.

The device for carrying condensate may comprise any suitable device. Forexample this may comprise piping between the heat exchangers sized andformed so that any condensate from the desuperheater heat exchanger iscarried by flow of gaseous working fluid into the inlet of the condenserheat exchanger. A typical arrangement man involve a standard “P” trap.

According to another aspect of the present invention the heat pumpsystem as described above is further provided with a liquid/gas heatexchanger arranged and configured so as to transfer heat from theworking fluid output from the condenser heat exchanger to the workingfluid input to the compressor.

The invention also covers a method of determining heated fluid mass flowrate and heated fluid entering temperature for a heat pump systemcomprising a desuperheater heat exchanger and a condensor heat exchangerconnected in series with a heated fluid flowing in series through thedesuperheater heat exchanger and condensor heat exchanger, comprisingthe steps of;

specifying a required heated fluid discharge temperature A, a requiredworking fluid condensing temperature B, a required desuperheater heatexchanger duty C, a required condenser heat exchanger duty D, atemperature difference between the working fluid and heated fluid atexit of the condenser heat exchanger F, and the specific heat capacityof the heated fluid G;

determining a heated fluid mass flow rate H according to the followingformula;$H = \frac{C}{G\left\lbrack {A - \left( {B - F} \right)} \right\rbrack}$

and then determining a heated fluid entering temperature E according tothe following formula;$E = {\left( {B - F} \right) - \left( \frac{D}{G \times H} \right)}$

The invention also covers a heat pump system for raising the temperatureof a fluid, comprising a desuperheater heat exchanger and a condenserheat exchanger connected in series, wherein required heat transferduties of the desuperheater heat exchanger and the condenser heatexchanger are determined so that a fluid passed in series through theseheat exchangers when operating at specified condensing and evaporatingtemperatures of a working fluid, becomes heated to a specifiedtemperature of at least the condensing temperature of the working fluid.

BRIEF DESCRIPTION OF DRAWINGS

Further aspects of the present invention will become apparent from thefollowing description which is given by way of example only and withreference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of a conventional heat exchangerconfiguration for hot gas cooling of a heat pump system.

FIG. 2 is a schematic diagram of a heat pump system according to a firstembodiment of the present invention.

FIG. 3 is a working fluid pressure-enthalpy diagram for the workingfluid cycle of the present invention.

FIG. 4 is a flow chart illustrating a method of determining parametersaccording to the present invention.

FIG. 5 is a heat transfer diagram for the present invention.

FIG. 6 is a schematic diagram of a heat pump system according to asecond embodiment of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

With reference to FIG. 2, there is shown a heat pump system generallyindicated by arrow 6 according to an embodiment of the invention. Theletters in FIG. 2 refer to locations around the circuit, which arediscussed later with reference to FIG. 3.

The heat pump system 6 is charged with a working fluid such as ahalogenated or natural type working fluid. Such working fluids includefor example: the HFC group (hydro-fluoro-carbons), the HC group(hydro-carbons), the FC group (fluoro-carbons), or blends composed ofthe preceding working fluids. Also, ammonia, water, carbon di-oxide andother inorganics may be used as the working fluid. With the presentembodiment HFC refrigerant R134a is used.

The heat pump system 6 comprises a compressor 7 for compressing theworking fluid, a desuperheating heat exchanger 8 provided with an inlet9 and outlet 10 for a heated fluid and an inlet 11 and outlet 12 for theworking fluid. The compressor 7 may be any suitable refrigerantcompressor. Preferably this would be of a hermetic or semi hermetic typewhere working fluid also cools the prime mover. In order to obtain thehigh pressures for the working fluid cycle, it is generally envisionedthat this would be a reciprocating type compressor of either single ormulti-stage configuration, however other compressors may also besuitable. Moreover, the motor for driving the compressor may be operatedat either a constant or a variable speed. Furthermore, two or morecompressors may be provided as required. Where economically indicated,and usually in situations with larger heating capacity requirements, theworking fluid pressure gradient between an evaporator 20 and thedesuperheater heat exchanger 8 may be reduced by replacing the singlestage compressor 7 with either multiple single-stage compressors set ina series arrangement so as to share the pressure gradient between themin such proportion as may be found desirable, or alternatively byselection of a multi-stage compressor or compressors to match the soughtduty.

The working fluid inlet 11 of the desuperheating heat exchanger 8 iscommunicated with an outlet 13 from the compressor 7. The system alsocomprises a condenser heat exchanger 14 provided with an inlet 15 andoutlet 16 for the heated fluid and an inlet 17 and outlet 18 for theworking fluid. The condenser heat exchanger working fluid inlet 17 iscommunicated directly with the superheater heat exchanger working fluidoutlet 12, and the condenser heat exchanger heated fluid outlet 16 iscommunicated directly with the superheater heat exchanger heated fluidinlet 9. Moreover, there is provided the evaporator 20 with an inlet 21communicated with the condensing heat exchanger working fluid outlet 18via the liquid side of a liquid/gas heat exchanger 22 and an expansionvalve 23, and an outlet 24 communicated with an inlet 25 to thecompressor 7 via the vapour side of the liquid/gas heat exchanger 22.The evaporator 20 is cooled by a coolant such as air or water, which isinput at a coolant inlet 26 and discharged at a coolant outlet 27.

The provision of the liquid/gas heat exchanger 22 serves to increase theoverall efficiency of the system by transferring heat from the workingfluid output from the condenser heat exchanger 14 to the working fluidinput to the compressor 25.

The arrangement of the heat pump system of FIG. 2 is aimed at satisfyingthe need to deliver water or other flows at both high temperatures andincreased flowrates without wastage, and moreover to enable a compactdesign. In this respect, while the heat exchangers may be anyconventional type of heat exchanger, it is found that brazed plate typeheat exchangers generally have more complete performance specifications,and hence the circuit specification can be more accurately predicted ifthis type of heat exchanger is used.

With the heat pump system 6 of FIG. 2, heated fluid (fluid to be heated)is applied in series flow, first through the condenser heat exchanger 14and then the desuperheater heat exchanger 8 in one undivided stream incounterflow to the working fluid. The heated fluid may be any suitablemedium for absorbing heal In the case where the heat exchangers areconnected to a recirculation system, it is generally envisioned thatthis would be water, or of an aqueous nature. Alternatively, in the caseof connection to a non-return application, this would be the particularfluid to be heated.

In designing this system, it is essential that the heated fluid flowshould fully serve the heat transfer requirements of both working fluidde-superheating and condensing, and that heated fluid temperatures becompletely applicable to serve the sought duties of the main process,which may, but not necessarily, be for a pasteurizing process.

Requirements of temperature, rate of heat transfer and the types ofworking fluid and heated fluid to be used form the starting points tocalculate the necessary heat transfer duties, and incorporate publisheddata from compressor manufacturers relative to their particular productat the selected condensing, evaporating and suction gas temperatures inthe formation of a balanced loop working fluid circuit as required ofany normal heat pump system.

FIG. 3 shows a working fluid pressure-enthalpy diagram for the workingfluid cycle of the present invention. The Y-axis is the absolutepressure in bar and the X-axis is the enthalpy in kJ/kg. The letters K,L, M, N, O, P, Q are the conditions at the various locations in thecircuit of FIG. 2. Here, K is the condition at the compressor inlet 25,L is the condition at the compressor outlet 13, M is the condition atthe desuperheater heat exchanger outlet 12, N is the condition at thecondenser heat exchanger outlet 18, O is the condition at the outletfrom the liquid/gas heat exchanger 22, P is the condition at theevaporator inlet 21, and Q is the condition at the evaporator outlet 24.The curved line in FIG. 3 shows the interface between saturated liquidand saturated vapour, and between dry vapour and superheated vapour. Inthis diagram it can be seen that the heat given up from the condensatebetween N and O through the liquid/gas heat exchanger is transferred tothe working fluid vapour between Q and K, thus improving the efficiencyof the heating cycle.

The coolant flows and temperatures available for use in the particularprincipal process, are determined for example according to the flowchart of FIG. 4. In step 1 the required heated fluid dischargetemperature A, the required working fluid condensing temperature B, therequired desuperheater heat exchanger duty C, the required condenserheat exchanger duty D, the working fluid to heated fluid temperaturedifference at exit of the condenser heat exchanger F, and the specificheat capacity of the heated fluid G are specified.

Then in step 2 the heated fluid flow mass flow rate H is determinedaccording to the following formula;$H = \frac{C}{G\left\lbrack {A - \left( {B - F} \right)} \right\rbrack}$

Subsequently in step 3 the heated fluid entering temperature E isdetermined according to the following formula;$E = {\left( {B - F} \right) - \left( \frac{D}{G \times H} \right)}$

Needless to say, appropriate changes to the many variables will allow oftailoring the resultant coolant temperatures to suit the principalprocess requirements of flow and temperature which may be beyond thatavailable from conventional systems.

Figures for typical calculations according to the above method are givenin Table 1. In these examples the heated fluid is water and the workingfluid is refrigerant R134a.

TABLE 1 Parameters Example 1 Example 2 A - Required heated fluiddischarge 85° C. 92° C. temperature B - Required working fluidcondensing 80° C. 78° C. temperature C - Required desuperheater heat ex-30 Kw 30 Kw changer duty D - Required condenser heat exchanger 70 Kw 70Kw duty E - Heated fluid entering temperature ° C. ° C. F - Temperaturedifference between 5K 3K working fluid and heated fluid at exit ofcondenser heat exchanger G - Specific heat capacity of heated 4.18kJ/kcal 4.18 kJ/kcal fluid H - Heated fluid mass flow rate kg/s kg/s

In the case of Example 1 $\begin{matrix}{{{Heated}\quad {fluid}\quad {mass}\quad {flow}\quad {rate}\quad H} = \quad \frac{30}{4.18\left\lbrack {85 - \left( {80 - 5} \right)} \right\rbrack}} \\{= \quad {0.718\quad {kg}\text{/}s}}\end{matrix}$ $\begin{matrix}{{{Heated}\quad {fluid}\quad {entering}\quad {temperature}\quad E} = \quad {\left( {80 - 5} \right) - \left( \frac{70}{4.18 \times 0.718} \right)}} \\{= \quad {51.7^{\circ}\quad {C.\quad {at}}\quad {condensor}\quad {inlet}\quad 15\quad \left( a^{\prime} \right.}} \\\left. \quad {{in}\quad {{FIG}.\quad 5}} \right)\end{matrix}$

In the case of example 2 $\begin{matrix}{{{Heated}\quad {fluid}\quad {mass}\quad {flow}\quad {rate}\quad H} = \quad \frac{30}{5 \times {4.18\left\lbrack {92 - \left( {78 - 3} \right)} \right\rbrack}}} \\{= \quad {0.422\quad {kg}\text{/}s\quad {flow}\quad {rate}}}\end{matrix}$ $\begin{matrix}{{{Heated}\quad {fluid}\quad {entering}\quad {temperature}\quad E} = \quad {\left( {78 - 3} \right) - \left( \frac{70}{4.18 \times 0.422} \right)}} \\{= \quad {35.3^{\circ}\quad {C.\quad {at}}\quad {condensor}\quad {inlet}}} \\{\quad {15\quad \left( {a^{''}\quad {in}\quad {{FIG}.\quad 5}} \right)}}\end{matrix}$

FIG. 5 is a heat transfer diagram for the present invention with theY-axis showing temperature in degrees Celsius and the X-axis showingtotal heat transfer in kW. Letters L, M, N refer to conditions at theaforementioned locations L, M, N in FIG. 2 for the working fluid. Linesa′, b, c′ and a″, b, c″ show conditions for the heated fluid for theabove examples 1 and 2 respectively. Points a′ and a″ correspond to theresultant heated fluid entering temperatures E, and points c′ and c″correspond to the required heated fluid discharge temperatures A. Inboth example 1 and example 2 points c′ and c″ are above the respectiverequired working fluid condensing temperatures B along the full andbroken lines M-N.

The ratio of L to M and M to N along the X-axis indicates the proportionof superheat heat transfer to latent heat heat transfer in the totalheat transfer process.

FIG. 6 shows a second embodiment of a heat pump fluid heating systemgenerally indicated by arrow 30 according to the present invention. Inthis figure, components having the same function as those in the firstembodiment of FIG. 2 are denoted by the same symbols.

The heat pump fluid heating system 30 is designed for use in aprocessing plant such as a milk pasteurizing plant. As such, the heatedfluid is circulated around a heating loop 32 incorporating a processheating load heat exchanger 33 by means of a circulation pump 34.Moreover, cooling fluid is circulated around a cooling loop 35 of afluid recirculation system incorporating the evaporator 20 and a processcooling load heat exchanger 36 by means of a circulation pump 37. In thecase of a pasteurizing plant the heating load would be the heat forheating milk to a pasteurizing temperature of around 72° C., and thecooling load would be that applied toward cooling the milk again.

With such an arrangement, the recirculation systems may be designed tosatisfy either the whole or part of the heating and cooling requirementsfor a pasteurizing or a thermalising plant or the like.

Another feature of the second embodiment, is that the desuperheater heatexchanger 8 is arranged so that the working fluid outlet 12 therefrom isbelow the inlet 17 to the condenser heat exchanger 14. In this case, inorder to carry condensate into the condenser heat exchanger inlet 17,piping 38 between the outlet 12 and the inlet 17 is sized and formed sothat condensate from the desuperheater heat exchanger 8 is carried byflow of the gaseous working fluid into the inlet 17 of the condensorheat exchanger 14. A suitable device for achieving this may be astandard “P” trap fitted into the piping.

Test results from a pilot-sized plant have proven predictability ofdesign, with constant and reliable 78° C. product hot water, and 4° C.cold water providing at least 37% of all required cooling.

The tested heat pump exhibited a 410% overall thermal efficiency, (4.10COP) using electricity as the motive power.

Whereas pasteurizing had been the original goal of the invention, suchother applications a thermalizing and general water heating are alsoforeseen.

It will be understood that all components utilized in the abovedescribed circuit are of conventional construction and are commerciallyavailable. The invention here relates not to the components, per se, butto the arrangement of such components in a circuit which can achievesufficient flows of high temperature fluid for use in processing plantssuch as for sterilizing, and pasteurizing.

INDUSTRIAL APPLICABILITY

The present invention has industrial applicability in that it provides aheat pump fluid heating system which enables a compact design, and whichcan achieve sufficient flows of high temperature fluid for use inprocessing plants such as for sterilizing, and pasteurizing. Moreover,the invention can obviate the need for; a fired steam or hot waterboiler, pressure vessel certification, safety surveys, water qualitytreatment and carbon emissions to the environment, and by the high COPfigures will avail considerable economies in energy costs.

Aspects of the present invention have been described by way of exampleonly and it should be appreciated that modifications and additions maybe made thereto without departing from the scope of the invention asdefined by the appended claims.

What is claimed is:
 1. A heat pump system for raising the temperature ofa single fluid to be heated, the single fluid to be heated referred toas a single heated fluid, comprising: a compressor for compressing aworking fluid; a desuperheater heat exchanger provided with an inlet andoutlet for said single heated fluid and an inlet and outlet for saidworking fluid, said working fluid inlet being communicated with anoutlet from said compressor; a condenser heat exchanger provided with aninlet and outlet for said single heated fluid and an inlet and outletfor said working fluid, said condenser heat exchanger heated fluidoutlet being communicated directly with said desuperheater heatexchanger heated fluid inlet, and said condenser heat exchanger workingfluid inlet being communicated directly with said desuperheater heatexchanger working fluid outlet; and an evaporator with an inletcommunicated with said condenser heat exchanger working fluid outlet,and an outlet communicated with an inlet to said compressor.
 2. A heatpump system according to claim 1, wherein said heat exchangers areadapted for connection to a non-return application.
 3. A heat pumpsystem according to claim 1, wherein said heat exchangers are adaptedfor connection to a fluid recirculation system.
 4. A heat pump systemaccording to claim 1, wherein said evaporator comprises a liquid cooledheat exchanger adapted for connection to a liquid recirculation system.5. A heat pump system according to claim 4, wherein said recirculationsystems satisfy either the whole or part of the heating and coolingrequirements for a pasteurizing or thermalising plant.
 6. A heat pumpsystem according to claim 1, wherein said desuperheater heat exchangeris arranged so that a working fluid outlet therefrom is below an inletto said condenser heat exchanger, and there is provided means forcarrying any condensate into said condenser heat exchanger inlet.
 7. Aheat pump system according to claim 6, wherein said condensate carryingmeans comprises piping between said heat exchangers sized and formed sothat any condensate from said desuperheater heat exchanger is carried byflow of gaseous working fluid into said inlet of said condenser heatexchanger.
 8. A heat pump system according to claim 1, wherein saiddesuperheater heat exchanger, said condenser heat exchanger and saidevaporator are brazed plate type heat exchangers.
 9. A heat pump systemaccording to claim 1, wherein said compressor is a reciprocatingcompressor.
 10. A heat pump system according to claim 1, wherein thereis further provided a liquid/gas heat exchanger arranged and configuredso as to transfer heat from the working fluid output from said condenserheat exchanger to the working fluid input to said compressor.
 11. A heatpump system according to claim 1, wherein said heated fluid issubstantially water.
 12. A method of determining heated fluid mass flowrate and heated fluid entering temperature for a heat pump systemcomprising a desuperheater heat exchanger and a condenser heat exchangerconnected in series with a heated fluid flowing in series through saiddesuperheater heat exchanger and condenser heat exchanger, comprisingthe steps of; specifying a required heated fluid discharge temperatureA, a required working fluid condensing temperature B, a requireddesuperheater heat exchanger duty C, a required condenser heat exchangerduty D, a temperature difference between said working fluid and heatedfluid at exit of said condenser heat exchanger F, and a specific heatcapacity of said heated fluid G; determining a heated fluid mass flowrate H according to the following formula;$H = \frac{C}{G\left\lbrack {A - \left( {B - F} \right)} \right\rbrack}$

and then determining a heated fluid entering temperature E according tothe following formula;$E = {\left( {B - F} \right) - \left( \frac{D}{G \times H} \right)}$


13. A heat pump system for raising the temperature of a fluid,comprising a desuperheater heat exchanger and a condenser heat exchangerconnected in series, wherein required heat transfer duties of saiddesuperheater heat exchanger and said condenser heat exchanger aredetermined so that a fluid passed in series through said heat exchangerswhen operating at specified condensing and evaporating temperatures of aworking fluid, becomes heated to a specified temperature of at least thecondensing temperature of said working fluid.